LARGE CRYSTALS, PORPHYRITIC TEXTURES, AND ORE DEPOSITS

Many ore deposits are associated with magmatic or hydrothermal systems that precipitate large crystals. For example, Cu and Mo deposits in convergent margins are usually associated with felsic porphyries, which are shallow-level intrusives characterized by cm-scale feldspar phenocrysts set within a fine-grained or micro-crystalline groundmass. In pegmatite Li deposits, Li is often found in the form of meter-sized spodumene crystals. Finally, many ilmenite-related Ti ores are associated with anorthosites, where meter-scale plagioclase crystals are frequent. Understanding how these large crystals grow, and in particular, how fast they grew, may be important for understanding ore genesis. Through a series of ongoing case studies, we are exploring the possibility of using the trace-element content of crystals as geospeedometers for crystal growth. For example, during rapid crystal growth, Ge and Al become enriched in quartz because diffusive timescales in growth media are long compared to crystal growth timescales. We show that miarolitic cm-scale quartz crystals in pegmatites formed within hours, suggesting that associated meter-sized crystals can potentially form in days. In another case study, we show P, Ti, and Al become enriched in olivine during rapid growth, allowing us to constrain the cooling rate of basaltic magma bodies. Our observations suggest that large crystals are large because they grew fast, not because they had longer times to grow. Extremely rapid growth rates must be facilitated by elevated transport rates through the growth media, suggesting that large crystals can only form in the presence of a free fluid phase. Release of volatiles and the partitioning of metals into the free volatile phase may explain why so many mineralizing systems in arcs are associated with large crystals. Anorthosites remain mysterious.